The present invention is generally in the field of tribology and specifically relates to applying lubricating compositions to surfaces to reduce the friction coefficient and wear on the surfaces.
When surfaces of a machine or device rub against each other, a friction force results, along with wear in the surfaces. The wear reduces the ability of the machine or device to function properly and efficiently. The frictional resistance can be reduced in a number of manners, such as by changing the structure of the surface, the material used, and/or by adding a lubricant between the surfaces. Lubricants separate the sliding surfaces by forming a film, and thereby reduce the frictional resistance and wear. However, under load increases and increased sliding speed, many lubricants break down. In the case of oil-based lubricants, the oil heats up with increases in speed and pressure, causing the lubricant to break down. Further, many oil-based lubricants are not suitable for industries, such as the food and beverage industry, which require the lubricant to not contaminate the food that is produced.
Water is an attractive alternative to conventional lubricating oils. It has ecological, health, safety, and economic advantages as a lubricant, as well as excellent heat-transfer properties. Therefore water serves as a coolant to the sliding surfaces. However, it has the disadvantage of a low-pressure coefficient of viscosity, which decreases its ability to support high loads. Nature solves this problem by coating the sliding surfaces in vivo with a “smart” material, cartilage, that changes in response to pressure and holds on the surface immobilized chains of biomolecules, which can function as boundary lubricants.
Most of the literature concerning lubrication by aqueous media is divided into articles dealing with (1) biological lubrication in the human body (Jay G D et al., J. Biomed. Mat. Res., 40(3): 414-418 (1998); Schwarz I M & Hills B A, Brit. J. Rheumatology, 37(1): 21-26 (1998); Smith A M A et al., Int. J. STD & AIDS, 9 (6): 330-335 (1998); Widmer M R et al., Tribology Letters, 10(1-2): 111-16 (2001); Xiong D S & Ge S R, WEAR, 250: 242-45 (2001)), (2) lubrication of ceramics (Basu B et al., WEAR, 250: 631-41(2001); Chen M et al., Tribology Letters, 11(1): 23-28 (2001); Francisco A et al., Tribology Transactions, 45(1): 110-16 (2002); Saito T et al., WEAR, 247(2): 223-30 (2001); Umehara N & Kato K, J. Japan. Soc. Tribologists, 42(11): 879-85 (1997)), geological effects involving water (Regenauer-Lieb K et al., Science, 294(5542): 578-80 (2001)), (3) hydraulic pumps (Wang D et al., Indust. Lubric. and Tribology, 53(5): 211-16 (2001)), and (4) oil-in-water emulsions (Ratoi-Salagean M et al., Proceedings Inst. Mech. Engin. Part J: Journal Engin. Tribology, 211(J3): 195-208 (1997) and Ratoi-Salagean M et al., Tribology Transactions, 40(4): 569-78 (1997)), or rubber tires on roads (Veith A G, Rubber Chem. Technol., 69(5): 858-73 (1996)). However, relatively few articles address the use of a single-phase aqueous lubricant containing a boundary lubricating additive for the lubrication of metal contacts.
Plaza S et al., WEAR, 249 (12): 1077-89 (2001) describes a polyoxyethylene diphosphate derivative that appears to show some anti-wear and friction reduction activity in aqueous solution. At a load of 5N, all samples tested showed friction coefficients at 5N of around 0.1. Lei H et al., WEAR, 252(3-4): 345-50 (2002) describes a fullerene-styrene sulfonic acid copolymer, which shows low (0.3) friction coefficient at the lowest loads reported (100 N). The wear scar is shown to be very sulfur rich after the wear tests. Duan B & Lei H. WEAR, 249(5-6): 528-32 (2001) reports the use of colloidal polystyrene as an additive to aqueous fluids such as triethanolamine aqueous solution and a water-soluble zinc alkoxyphosphate (OPZ) solution. The addition of colloidal polystyrene to an aqueous base fluid appears to have a beneficial effect on the wear behavior of steel, as demonstrated by the maximum non-seizure load. However, the wear-scar diameter is not significantly reduced compared to the wear-scar diameter using a colloid-free solution, and no friction-reducing behavior is disclosed. Hollinger S et al., Tribology Letters, 9(3-4): 143-151 (2000) reports the use of vesicular and lamellar systems, suspended in phosphate-containing solutions, which appear to reduce friction in interfaces between brass and tungsten.
Multifunctional copolymers described in U.S. Pat. Nos. 5,462,990 and 5,627,233 and WO 98/47948 all to Hubbell et al. have been used in as surgical sealants and in analytical devices. U.S. Pat. Nos. 5,462,990 and 5,627,233 to Hubbell et al. discloses multifunctional polymeric materials for use in inhibiting adhesion and immune recognition between cells and tissues. The materials include a tissue-binding component (polyionic) and a tissue non-binding component. In particular, Hubbell discloses various PEG/PLL copolymers, with molecular weights greater than 300, with structures that include AB copolymers, ABA copolymers, and brush-type copolymers. These polymers are being commercially developed for use as tissue sealants and to prevent surgical adhesions. WO 98/47948 by Hubbell et al. describes grafted polyionic copolymers that are able to attach to biological and non-biological samples in order to control cell-surface and cell-cell and tissue-surface interactions in biomedical applications. WO 00/065352 by Hubbell et al. describes polyionic coatings in analytical and sensor devices, which promote specific recognition of a target analyte and at the same time minimize non-specific adsorption of other molecules in the sampling solution. However, these materials have never been used as lubricants.
There is a need for improved lubricating compositions. In particular there is a need for compositions which can reduce friction in metal oxide surfaces.
Therefore, it is an object of the invention to provide a stable polymeric material that can be added simply, quickly and cost-effectively to an aqueous medium to produce an environmentally friendly, aqueous lubricant.
It is a further object of the invention to coat metal oxide surfaces and other charged surfaces with a lubricating composition to reduce the friction coefficient and wear on the surfaces.